Due to size and weight reduction of electronic devices and their improved performance, the amount of heat that generates in a semiconductor device tends to increase. In recent years in a motor controlling field, a power semiconductor having a large band gap such as silicon carbide or gallium nitride has been developed for applications in which high voltage or high current is required such as an electric power device, a vehicle, or a machine tool. Such a power semiconductor can operate at a higher temperature compared with a conventional silicon semiconductor because the semiconductor element itself has high heat resistance.
In order to take advantage of such characteristics of the power semiconductor, a bonding material having an excellent thermal conductivity is required, which can efficiently release the heat generated in operation of the semiconductor. As described in Patent Literature 1, in recent years, a paste containing nano-sized metal fine particles has been studied as a bonding material having an excellent thermal conductivity. In a bonding method using such a paste, a sintering phenomenon of metal particles is utilized for the bonding.
However, in the bonding method utilizing the sintering phenomenon of metal particles as described in Patent Literature 1, a shrinkage stress is generated by a sintering shrinkage caused by a sintering reaction, and remains in a bonded part. Furthermore, when members to be bonded have largely different thermal expansion coefficients, a higher thermal stress is applied to the bonded part during a cooling-heating cycle at the operation of the semiconductor device, which leads to problems such as generation of cracks in semiconductor elements or in the bonded part, or exfoliation of the bonded part.
As a method for reducing such a thermal stress applied to a bonded part, Patent Literature 2 discloses a conductive bonding material comprising metal fine particles made of a first metal, and metal-coated resin particles having a diameter larger than that of the metal fine particles and coated with a second metal. Patent Literature 2 explains that, with such a bonding material, the stress applied to a bonded part between a semiconductor element and a circuit pattern can be reduced.
Patent Literature 3 discloses a semiconductor device including a metal lead frame and a semiconductor chip in which the semiconductor chip and the metal lead frame are bonded by a porous joint layer in which conductive particles containing Ag as a bonding material are bonded to each other, and at least a part of air spaces among the conductive particles is filled with a thermosetting resin.
On the other hand in Patent Literature 4, there is disclosed a semiconductor device including a semiconductor chip and an insulating substrate in which the insulating substrate comprises the semiconductor chip mounted thereon and an electrode plate provided on its surface. Patent Literature 4 discloses that, in a sintering metal layer that bonds the semiconductor chip and the electrode plate, a porosity of an end part of the layer is set to be higher than a porosity of the center part of the layer, and thereby a stress applied to the end part of the bonding layer can be relaxed.